Police Chief Paul E. Johnson on Thursday fired a University security guard who said he was harassed by his supervisors and assaulted by a colleague.

The guard, a Russian citizen who has requested anonymity due to fears that his immigration status could be jeopardized, said he was fired for allegedly instigating a fight and filing a false police report.

The fired guard has already filed a grievance with his union, and he said he would take his case to arbitration if he could not win his job back from the University. The guard said he would not rule out the possibility of suing Harvard.

Lt. John F. Rooney, who investigated last month's fight between the two guards, said his probe was "not completely conclusive" as to who had instigated the assault.

But Johnson, in rendering his decision, appeared to draw the conclusion that the fired guard was responsible for the incident. The other guard was not suspended or other wise disciplined, according to department sources.

The fired guard had said he was hit twice and was threatened by the other guard on January 21.

He filed the report that Johnson determined to be false on January 25, and the other guard filed a police complaint against the fired guard the next day. Manager of Operations for Security Robert J. Dowling, the lone witness to the incident, also was interviewed by police.

Johnson did not return a telephone call yesterday.

Dowling had suspended the fired guard indefinitely without pay after the January 25 incident, and the guard has been unemployed since then. The guard said he was not given any reason for the suspension.

Dowling officially informed the guard of the department's decision to fire him during a meeting on Thursday afternoon.

Last May, the fired guard publicly charged that Dowling condoned repeated harassment by security supervisor Thomas F. Henaghan. He also said that when he complained to Johnson about the harassment, he was suspended.

Henaghan and other department officials denied those charges, saying that the guard warranted close supervision because of a history of disciplinary problems.

The fired guard was terminated once before, in January 1989, for allegedly sleeping on the job. He won his job back that time after filing a union grievance.

General Counsel Margaret H. Marshall last month asked all guards with knowledge of harassment or discriminatory practices in the security department to come forward and consent to interviews.

Marshall yesterday refused to comment on the firing, but said the fired guard could still choose to be interviewed by investigators